To date, as for video signal transmission in a front-door visual phone or the like, cable transmission using coaxial cables or parallel cables has been adopted. However, because of the facility of fitting work, adoption of a wireless video transmission system, in which a parent unit and a child unit are connected by utilizing a radio wave, is considered.
Further, as a method for receiving standard television broadcasting as well as performing wireless video transmission, there has been studied a method in which one channel is selected from vacant channels of a television and video is transmitted by utilizing a minute-power wave. This method uses an RF converter for generating a standard television signal and an RF tuner for receiving the standard television signal.
Further, as another means, there has been studied a method in which an RF tuner is used for receiving broadcasting, and a digitized video signal is transmitted by using a small-power radio transmitter-receiver, in combination with the data compression/decompression technique.
FIG. 11 shows a video transmission device as an example of a conventional transmission apparatus using a minute-power wave.
In FIG. 11, 801 denotes a transmitter for transmitting a video signal, 809 denotes a video source for outputting the video signal to the transmitter 801, 802 denotes an RF converter for generating a standard television signal, 803 denotes a channel switch for selecting a transmission frequency of the RF converter 802, and 804 denotes a transmission antenna of the transmitter 801. Further, 805 denotes a receiver for receiving the video signal, 806 denotes a receiving antenna of the receiver 805, 807 denotes an RF tuner for receiving the standard television signal, 808 denotes a video reproduction circuit for reproducing the video signal demodulated by the RF tuner 807, and 810 denotes a display unit for displaying the video from the receiver 805.
Next, the operation will be described. In the above-described structure, in the transmitter 801, a frequency signal selected by the channel switch 803 is modulated with the video signal from the video source 809 by the RF converter 802. Then, in the transmitter 801, the modulated signal is transmitted through the transmission antenna 804. On the other hand, in the receiver 805, the video reproduction circuit 808 reproduces the video signal from the signal selectively received by the receiving antenna 806 and the RF tuner 807, and the display unit 810 displays the video.
In such wireless transmission utilizing a radio wave, since the wave is a limited resource, it is appropriate to utilize a minute-power wave in a place where the available range is limited, such as in a house. The minute-power wave is a wave which hardly affect a wireless apparatus such as a television receiver in a house. However, the reachable range of the minute-power wave is short and, therefore, the distance between a parent unit and a child unit which use the minute-power wave is unfavorably limited.
The present invention is made to solve the above-described technological problem of the conventional apparatus, and it is an object of the present invention to provide a transmission apparatus which can establish a transmission path between a master station and a slave station, such as a parent unit and a child unit, which are placed apart by a distance longer than the reachable range of a minute-power wave.
Further, in the conventional transmission apparatus, since the transmission power is at the minute-power level and the reception sensitivity is low even in short-distance transmission, the influence of multi-path is considerable.
Furthermore, there is a risk that the minute-power radio apparatus using the frequency band of the standard television broadcasting will adversely affect reception of existing television broadcasting. Moreover, the minute-power radio apparatus will become unavailable when being affected by a strong existing broadcast wave.
Furthermore, in order to implement the above-described method in which an RF tuner is used for receiving broadcasting and a digitized video signal is transmitted using a small-power radio transmitter-receiver in combination with the data compression/decompression technique, the following apparatuses are required: an RF tuner, an AD converter, a DA converter, a compression/decompression processing circuit, a small-power wireless transmitter, and a small-power wireless receiver. Therefore, it is difficult to realize this method in regard to cbst.
The present invention is made to solve the above-described problems of the conventional apparatus, and it is an object of the present invention to provide a transmission apparatus which hardly become unavailable due to the influence of an existing broadcast wave even when it transmits information by using the frequency band of the standard television broadcasting, and which can be easily realized in regard to cost, and a transmission method using the transmission apparatus.